1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image-forming apparatus using the same.
2. Description of the Related Art
To date, in an optical scanning apparatus, light beams modulated and emitted from a light-source unit are periodically deflected by a light deflector according to image signals, and converged on a plane of a photosensitive recording medium as a spot by an imaging optical system having an fθ characteristic such that images are recorded on the plane.
Recently, such imaging optical systems have been required to be faster and more compact as apparatuses such as laser-beam printers, digital copiers, and multifunctional printers increase in speed and decrease in size.
As one measure for enhancing the speed, an overfilled optical system (OFS) has been used. In the OFS, the width of incoming beams incident on a deflection plane of a light deflector (rotatable polygon mirror) is larger than the width of the deflection plane in a main scanning section. Accordingly, the diameter of the light deflector can be small and the number of planes thereof can be increased, and thus the speed of the light deflector can be enhanced.
According to this feature of the OFS, the incoming beams incident on the deflection plane of the light deflector and scanning beams deflected from the deflection plane and reaching a surface to be scanned are required to be spatially separated in a sub-scanning section.
Also, when a plurality of light beams are deflected from an identical deflection plane of a light deflector and individually emitted to a plurality of planes to be scanned in an underfilled optical system (UFS), the plurality of light beams are required to be spatially separated in the sub-scanning section after the deflection.
In order to spatially separate the light beams, for example, the direction of the incoming beams incident on the deflection plane of the light deflector is inclined with respect to the deflection plane in the sub-scanning section.
When the optical structure of the optical scanning apparatus becomes compact, the length of the optical path for spatial separation becomes short. Therefore, the inclination of the incoming beams incident on the deflection plane of the light deflector defined in the sub-scanning section is required to be larger with respect to the deflection plane. Moreover, the maximum scanning angle is required to be larger such that the length of the optical path from the deflection plane of the light deflector to the surface of a photosensitive drum (the surface to be scanned) is reduced.
However, when the inclination of the incoming beams and the scanning angle are increased as described above, the light beams obliquely incident on the deflection plane of the light deflector in the sub-scanning section draw a conical plane due to the deflection scanning of the light deflector (conical scan), resulting in the following two major problems.
A first problem is that the light beams obliquely incident on the deflection plane of the light deflector in the sub-scanning section draw a conical plane due to the deflection scanning of the light deflector, and thus, the curved scanning beams are incident on an fθ lens (imaging lens) disposed in the center of the oblique incoming beams such that a curved scanning line also appears on the surface of the photosensitive drum serving as the surface to be scanned. This is the problem of the curvature of the scanning line caused by the incoming beams obliquely incident on the deflection plane of the light deflector in the sub-scanning section.
A second problem is deterioration in imaging performance since the scanning beams that are deflected from the deflection plane and reach the surface to be scanned draw a conical plane. When the main scanning direction in the incident optical system can be defined as a main scanning direction of the light beams, the angle of the deflected light beams inclined with respect to generatrices (main scanning direction) of the fθ lens in the main scanning direction is increased with the scanning angle. That is to say, since directions (main scanning direction and sub-scanning direction) of refracting power to be given to the light beams by the lens surfaces are deviated (rotated) in areas with large scanning angles, the shape of the spot on the surface of the photosensitive drum is distorted to form, for example, a star shape. This is the problem of the deterioration in the imaging performance caused by the scanning beams drawing a conical plane.
When an optical scanning apparatus having the two problems described above is used for an image-forming apparatus, formed images are considerably deteriorated.
To date, various optical scanning apparatuses for solving these two problems have been discussed (for example, U.S. Pat. No. 6,141,118).
In U.S. Pat. No. 6,141,118, the optical axis of a toric lens used in an imaging optical system and light beams deflected by a light deflector are disposed collinear with respect to each other in the sub-scanning section, and both surfaces of the toric lens are formed such that generatrices formed by connecting the vertexes of meridians of each lens surface are curved in the sub-scanning direction. In this manner, the deterioration in the imaging performance and the curvature of the scanning line are corrected.
In this method, the generatrices of both the lens surfaces of the toric lens curved in the sub-scanning direction produce an effect equal to a case where a cylindrical lens having power in the sub-scanning direction is rotated around the optical axis of the toric lens. By action of this effect of rotating the cylindrical lens around the optical axis of the toric lens, the influence of deviation (rotation) in the directions (main scanning direction and sub-scanning direction) of the refracting power to be given by the lens surfaces to the light beams deflected by the conical scan is cancelled, and the deterioration in the imaging performance is corrected or error reduced.
However, when the lens surfaces of the toric lens are formed such that the generatrices are curved in the sub-scanning direction, the shapes of the lens surfaces become complicated. Therefore, the lens surface having a high degree of effectiveness of reduction in the deterioration in the imaging performance can be formed such that the generatrix is curved in the sub-scanning direction.
Moreover, in U.S. Pat. No. 6,141,118, the optical axis of the toric lens of the imaging optical system is inclined with respect to the incoming beams that are incident on the incident plane of the toric lens in the sub-scanning section such that the deterioration in the imaging performance and the curvature of the scanning line are regulated. However, when the maximum scanning angle (maximum angle of view) or the inclination of the incoming beams incident on the deflection plane of the light deflector with respect to the deflection plane defined in the sub-scanning section is large in the imaging optical system, the deterioration in the imaging performance and the curvature of the scanning line on the surface to be scanned over the entire range of image heights are difficult to sufficiently regulate by changing the inclination of the toric lens in the sub-scanning section.